Fall 2013
EELE 250 Circuits, Devices, and Motors
Lab #6: Op Amps, Part 1
Scope:
Study basic Op-Amp circuits: voltage follower/buffer and the inverting configuration.
Home preparation:
•
•
•
•
Review Hambley chapter 14.
Read through the experiment and plan out each step.
Create tables in your notebook with the calculated values and space to enter the
measured results for the experiment.
Prepare the calculated results for the circuits you will be measuring in the lab, fill
out the prelab sheets, AND write the results in your lab notebook so you can
refer to them during the lab while your prelab is being graded.
Laboratory experiment:
This figure shows the 8-pin DIP (Dual Inline
Package) version of a type 741 operational
amplifier (op-amp) with its terminals labeled.
The pins are labeled 1 through 8,
counterclockwise, using the notch or stripe on
the top of the package to indicate the
orientation.
Note that there is no “ground” pin on the op
amp package. The op amp’s differential
inputs “float” with respect to ground.
Sometimes we will connect one of the inputs
to ground, but there is no pin that is always
grounded.
+12 V
-12 V
The positive (+12V) and negative (-12V) power supply voltages need to be connected to share
the common ground point. The dual output power supplies in the lab can be switched from
independent operation to series mode. When in series mode the negative end of the right
supply and the positive end of the left supply are internally connected. If you connect this middle
point with a jumper cable to chassis ground, you can then set the dual supplies so that the
rightmost terminal is +12 volts and the leftmost terminal is -12 volts with respect to the common
middle point ground. Note that when switched to series mode some of the lab power supplies
will go into “slave” mode so that one knob actually controls both supplies.
The Offset Null (pins 1 and 5) will not be used in this experiment. Those pins must be left
unconnected.
1
EELE 250 Lab #6
Fall 2013
Experiment
1. Using the DMM, carefully adjust the bench DC power supply to produce +12V and -12V
with respect to a common ground  see the comments above in the introduction, and ask
your lab TA to check your configuration.
2. Next build the circuit shown in Figure 1 on your breadboard.
Choose resistors with value close to the nominal values listed. This
chain of series resistors is a voltage divider, assuming all of the
resistors conduct the same current. You will use this circuit to
create several voltages to use in the rest of the experiment.
Measure and record the voltages in column 2 of Table 1 below.
6.2 k
3. Note that if a new circuit connection is made that draws or inserts
current into any of the nodes a, b, c, or d, the corresponding node
voltages will change due to the change in the overall current. Try
this by temporarily connecting another 1 kΩ resistor between node
b and ground. Re-measure the node voltages and record them in
column 3 of Table 1.
Pre-lab
predicted voltage
Table 1
Measured
in lab
(part 2)
Measured with 1kΩ
resistor inserted from b
to ground (part 3)
Va
Fig. 1
Vb
Vc
Vd
When done with step 3, remember to remove the 1kΩ resistor you attached between node
b and ground. Leave the rest of the voltage divider connected--you will use it in the next
parts of the experiment.
4. TURN OFF THE BENCH SUPPLY and carefully insert one of your 741 op amps into the
breadboard. The best way to set up this circuit is to CAREFULLY insert the op amp DIP so
that it straddles one of the grooves in your breadboard and the pins slide into the
breadboard holes: this allows connections to be made separately to each pin on the
amplifier package. Refer to the diagram in the introduction to identify which pin on the
package corresponds to which terminal on the schematic diagram. Once the chip is
inserted, connect the power supplies
 +12 V
Vin

and a jumper wire between pins 2
Vout
and 6 to create the voltage follower
circuit, or voltage buffer, with a 1 kΩ


load, as shown in Figure 2.

-12 V
1 kΩ
Fig. 2
2
EELE 250 Lab #6
Fall 2013
5. Turn on the bench power supply with the +12 and -12 voltages powering the op amp, and
in Table 2 record Vin and Vout when the voltage follower circuit is connected one by one to
nodes a, b, c, and d of the voltage divider.
With Vin
connected
to
Table 2
Pre-lab
predicted Vout
Vin measured
in lab
Vout measured
in lab
(part 5)
Va
Vb
Vc
Vd
In your notebook, comment on the result of using the voltage follower to drive the 1 kΩ
resistor, compared to what happened in part 3 when the 1 kΩ load was connected directly to
the nodes in the divider circuit. Specifically, note that the voltage follower draws negligible
current from the voltage divider, but provides sufficient current to drive the 1kΩ load.
6. TURN OFF THE POWER SUPPLY.
7. Now take a second op amp chip from your kit and insert it also onto the breadboard. Make
external connections to the +12 and -12 power supply for the second chip and attach
resistors to create an inverting amplifier configuration driven by the previous voltage
follower circuit, as shown in Figure 3.
Fig. 3

+12 V




+12 V



-12 V


-12 V
voltage follower
(chip 1)
inverting amp
(chip 2)
3
EELE 250 Lab #6
Fall 2013
8. Turn on the power supply so that both chips are active, and in Table 3 record Vin and Vout
when the two-chip circuit is connected one by one to nodes a, b, c, d of the voltage divider.
With Vin
connected
to
Table 3
Pre-lab
predicted Vout
Vout measured
in lab
(part 8)
Vin measured
in lab
Va
Vb
Vc
Vd
9. Turn off the power supply and disconnect the voltage follower output from the inverting
amplifier.
10. Turn the supply back on, and make the measurements once more using just the inverting
amplifier configuration alone (input via the 1kΩ resistor) (Fig. 4) connected one by one to
nodes a, b, c, d of the voltage divider and record your results in Table 4.
Fig. 4

Vin

Vout

With Vin
connected
to
Table 4
Pre-lab
predicted Vout
Vin measured
in lab
Vout measured
in lab
(part 10)
Va
Vb
Vc
Vd
Comment on the result of having the voltage divider circuit connected to the inverting amplifier
configuration without the voltage follower circuit in between.
4
EELE 250 Lab #6
NAME:
PRELAB SHEETS
Perform the calculations before coming to lab, and show a summary of your work.
Voltage divider
1. For the circuit of Fig. 1, determine the node voltages a, b, c, d with
respect to ground, and place the values in column 1 below. Explain
your work.
6.2 k
2. Next, with another 1kΩ resistor added between node b and ground,
repeat your calculation of the node voltages a, b, c, d and place the
values in column 2. Explain the effect of adding the 1kΩ resistor.
Pre-lab
predicted node
voltage
Pre-lab predicted node
voltage with a 1kΩ
resistor inserted from b
to ground
Va
Vb
Fig. 1
Vc
Vd
Voltage follower
3. If the input to the voltage follower circuit of Fig. 2 is connected one by one to nodes a, b, c,
d, of the five resistor circuit of Fig. 1, what is the expected output voltage?
Pre-lab
predicted Vin
Pre-lab predicted Vout
from voltage follower
Va
Vb
Vc
Vd
Prelab
EELE 250 Lab #6
NAME:
Voltage follower and inverting amplifier
4. If the input to the two-amplifier circuit of Fig. 3 is connected one by one to nodes a, b, c, d,
of the voltage divider, what is the expected output voltage? Explain.
Pre-lab
predicted Vin
Pre-lab predicted Vout
from Fig. 3 circuit
Va
Vb
Vc
Vd
Inverting amplifier without voltage follower
5. If the input to the inverting amplifier circuit of Fig. 4 is connected one by one to nodes a, b, c,
d, of the voltage divider, what is the expected output voltage? Explain your work.
Pre-lab
predicted Vin
Pre-lab predicted Vout
from Fig. 4 circuit
Va
Vb
Vc
Vd
Prelab